1. Field of the Invention
This invention relates generally to filters for use in subterranean environments, such as in oil or gas wells, water wells, and geothermal wells, and in other applications in which it is desired to remove a liquid or gas from the ground without bringing soil particulates, such as sand or clay, up with the liquid or gas.
2. Description of the Related Art
Filters are frequently used in subterranean wells in order to remove particulates from liquids or gases produced by the wells. Typical particulates which need to be filtered out are sand and clay, and for this reason, well filters for this purpose are often referred to as sand screens.
A possible mode of failure of a well filter is one in which portions of the filter are eroded through by sand particles in the fluid being filtered. When the flow of fluid through the filter is slow, the erosion of the filter by the sand particles is quite gradual. However, under some circumstances, the fluid with the well is directed against the well filter at a high velocity, and the sand particles in the fluid produce an abrasive effect on the filter which is similar to sand blasting. Such high velocity fluid flows can occur when portions of the formation in which a well is formed become plugged up, so that fluids are forced through the unplugged portions at a higher velocity than normal. The sand in a high velocity fluid flow is able to abrade not only through the filter medium of a well filter but also through solid steel portions of the filter. Thus, high velocity flow of sand-containing fluids within a well can significantly shorten the lifespan of a well filter.